The invention relates to an internal-combustion engine, particularly to an Otto engine, which can be operated in a homogeneous charge compression ignition mode at least under certain operating conditions, as well as to a process for controlling a correspondingly constructed internal-combustion engine.
Currently, many efforts are being made to reduce fuel consumption at partial load on an engine while simultaneously maintaining extremely low pollutant emissions. In this case, unthrottled Otto-engine-type stratified combustion processes exhibit great potential for reducing consumption. In the stratified combustion process or the lean operation, fuel injection into the cylinder is not carried out until just before the ignition point-in-time so that, at the point-in-time of the ignition, despite a very low quantity of injected fuel, an ignitable combustion mixture forms around the spark plug. The flame front generated by means of the spark plug spreads from there into the entire combustion chamber. The conversion speed therefore depends on the flame speed. However, the problem of these stratified lean concepts is that a high-expenditure exhaust gas aftertreatment is necessary for reducing nitrogen oxides.
The above-mentioned problem can be avoided by a so-called homogeneous charge compression ignition process—also called HCCI—which can be used under certain operating conditions of the internal-combustion engine. In the case of the HCCI process, in the homogeneous charge compression ignition mode of the internal-combustion engine, the ignition does not—as in conventional processes—take place at the spark plug in a punctiform manner but rather in the entire combustion chamber almost simultaneously by auto-ignition of the combustion mixture because, during the compression (or in the course of the process), the cylinder charge reaches auto-ignition conditions and thereby auto-ignites. The combustion is therefore not initiated by a punctiform external ignition source but is determined only by the ignition conditions in the combustion chamber. The conditions required for this purpose are ensured, for example, by the return of hot residual gas, by a preheating of the intake air or by an increase of compression. On the one hand, this combustion process has a high potential for reduced fuel consumption and, on the other hand, the NOx crude emission is lowered so far in large operating ranges that a lean operation becomes possible also without a high-expenditure exhaust gas aftertreatment.
However, in the case of the above-mentioned combustion processes, it is problematic that the combustion in the homogeneous charge compression ignition mode, in contrast to the external ignition, where the ignition point-in-time functions as a dominant control parameter, cannot be controlled by means of a singularly applicative parameter. Because of the relatively slowly expanding combustion front in the case of an ignition of the combustion mixture by means of a conventional spark plug, the course of the HCCI combustion process can only be marginally influenced by such ignition systems; i.e. such ignition systems are not suitable for controlling the course of the combustion in the entire operating range of this combustion process.
An arrangement and a method for controlling a homogeneous charge compression ignition process is known from U.S. Pat. No. 6,986,342 B2, where by means of a corona discharge device, which may be arranged either in the intake port of the cylinder or the combustion chamber of the cylinder, an ionization of the combustion mixture can be carried out. The point-in-time of the combustion can be influenced by the degree of ionization. Furthermore, a triggering unit is disclosed which permits a triggering of the auto-ignition.
It is an object of the invention to provide an internal-combustion engine and a corresponding process by which a controlled and rapid ignition can be achieved in the homogeneous charge compression ignition mode.
This and other objects are achieved by an internal-combustion engine, particularly a direct injection (Otto) internal-combustion engine, possibly having a supercharging device, which can be operated in a homogenous charge compression ignition mode at least under certain operating conditions. In this case, as described above, the combustion mixture present or formed in the combustion chamber reaches auto-ignition conditions as a result of the compression as well as the occurring chemical processes, so that the chemical conversion starts either isochronously or typically in fast succession in the entire combustion chamber.
In order to be able to actively ignite the combustion mixture outside the homogeneous charge compression ignition mode, an ignition system has to be provided in the case of Otto internal-combustion engines, which ignition system triggers the conversion of the combustion mixture as a function of the desired point-in-time of the ignition. According to the prior art, conventional spark plugs are used for this purpose, which generate a spark that ignites the combustion mixture in the immediate surroundings of the spark. As a function of the speed of the flame front propagation, the combustion mixture is successively ignited in the entire combustion chamber.
The invention recognizes that, in addition to conventional spark plug ignitions systems, so-called corona ignition systems are known. In the case of corona ignition systems, a very high voltage in connection with a very inhomogeneous electric field distribution is generated at an electrode tip in the combustion chamber, for example, by way of a Tesla coil, which is resonated by way of a high-frequency alternating voltage. As a result, the electric field locally becomes sufficiently high in order to generate an incomplete spark breakthrough (corona) in the combustion chamber, in which electrons are pulled or pushed out of the atom or molecule cores. No complete conductive ion conduit from the high-frequency/high-voltage electrode to the mass is thereby generated. Depending on the frequency of the excitation, only a slight heating of the generated so-called non-thermal corona plasma takes place. Radicals are formed by which a chemical chain reaction is started in the combustion chamber.
In order to achieve a controlled ignition in the homogeneous charge compression ignition mode in the case of internal-combustion engines that can be operated in a homogeneous charge compression ignition mode at least under certain operating conditions, a corona ignition system is used as a triggering unit for triggering or activating this auto-ignition before the auto-ignition conditions are reached. As a result of the generated corona discharge, an initialization of the spatial auto-ignition takes place, which initialization is targeted with respect to time, in that radicals form along the corona threads, whereby the reactivity of the combustion mixture is increased locally. A very fast ignition is thereby initialized along the corona threads, and thus the auto-ignition (for example, by a pressure-induced temperature increase and radical chain explosion) is triggered in the residual areas of the combustion chamber.
In order to save unnecessary costs and space, this corona ignition system, especially in the case of Otto internal-combustion engines, according to the invention, can be further developed and arranged such that it can also be used for the (external) ignition of the combustion mixture especially by generating a so-called non-thermal plasma outside the homogeneous charge compression ignition mode. In this case, a conventional ignition system (spark plug system) for the active external ignition of the combustion mixture outside the homogeneous charge compression ignition can be eliminated.
Analogous to the internal-combustion engine according to the invention, the process according to the invention for controlling an internal-combustion engine during a homogeneous charge compression ignition mode (as described above), in the case of which the auto-ignition can be triggered, before the auto-ignition conditions have been reached, by means of a triggering unit, is characterized in that the triggering unit is constructed as a corona ignition system. The corona ignition system triggers or activates the auto-ignition during the homogeneous charge compression ignition mode, particularly by generating a so-called non-thermal plasma.
In the case of the current state of the art, in the homogeneous charge compression ignition mode, the point-in-time of the auto-ignition is, as a rule, set by the targeted control of the course of the process (for example, variation of the mixture temperature and injection point-in-time, etc.). However, because of varying marginal conditions of the course of the process, this point-in-time cannot be set in a precise manner. As a result, in the case of the application of the corona ignition system according to the invention, the combustion mixture is transformed only into a condition shortly before the auto-ignition threshold, so that, by means of a triggering by the corona ignition system (particularly by generating a corona discharge), the combustion can be initiated at a precise point-in-time.
The corona discharge for activating the auto-ignition in the homogeneous charge compression ignition mode at the precise point-in-time can, in this case, be generated as a function of the desired optimal point-in-time of the ignition. For a controlled course of the combustion, the ignition voltage, the point-in-time of the ignition, and/or the duration of the ignition of the corona ignition system can be predefined in a variable manner for triggering or for activating the auto-ignition in the homogeneous charge compression ignition mode, and the ignition voltage can be modulated during the corona discharge. Thus, for example, as required, a targeted retard of the start of the conversion into the expansion cycle can be caused.
The corona ignition system may advantageously also be ignitable several times within one working cycle, so that, for the initiation or control of the conversion, as required, a targeted multiple ignition can be carried out.
In a further advantageous development of the internal-combustion engine according to the invention and of the process according to the invention, the corona ignition system can additionally be operated as an ignition system for igniting the combustion mixture outside the homogeneous charge compression ignition mode and/or as a mixture processing unit for the radical-chemical processing of the combustion mixture and/or as an ignition unit for igniting the combustion mixture present in the combustion chamber during an intermediate compression phase. The operative range for a radical-chemical processing may, for example, be from the “exhaust closes” point-in-time of the preceding cycle to the (auto)-ignition point-in-time of the current cycle. An operation of the corona ignition system as an ignition unit for igniting the combustion mixture present in the combustion chamber during an intermediate compression phase could take place, for example, during the time period when a negative valve overlap is present. This time period is between “exhaust closes” and “intake opens”. Thus, for expanding the operating range of the spatial auto-ignition, fuel can be injected into the residual gas (with residual oxygen) during this time period and the mixture can subsequently be ignited by the corona discharge.
The process according to the invention as well as its advantageous embodiments can be implemented by means of an algorithm or a corresponding module arrangement in a control device provided for that purpose, particularly in an engine control unit.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawing.